A laboratory fume hood is a ventilated enclosure where harmful materials can be handled safely. The hood captures contaminants and prevents them from escaping into the laboratory by using an exhaust blower to draw air and contaminants in and around the hood's work area away from the operator so that inhalation of and contact with the contaminants are minimized. Access to the interior of the hood is through an opening which is closed with one or more sashes which may slide vertically, horizontally, or in both directions to vary the opening into the hood.
The velocity of the air flow through the hood opening is called the face velocity. The more hazardous the material being handled, the higher the recommended face velocity. Typical face velocities for laboratory fume hoods are 75 to 150 feet per minute (fpm), depending upon the application.
When an operator is working in the hood, the sash or sashes are opened to allow free access to the materials inside. The sash or sashes may be opened partially or fully, depending on the operations to be performed in the hood. While fume hood and sash sizes vary, the opening provided by a fully opened sash is typically on the order of ten square feet. Thus the maximum air flow which the blower must provide is typically on the order of 750 to 1500 cubic feet per minute (cfm).
The sash is closed when the hood is not being used by an operator. It is common to store hazardous materials inside the hood when the hood is not in use, and a positive airflow must therefore be maintained to exhaust contaminants from such materials even when the hood is not in use and the sash is closed.
It is important that the face velocity be kept as constant as possible. The minimum acceptable face velocity is determined by the level of hazard of the materials being handled, as discussed above. Too high a face velocity may cause turbulence, however, which can result in contaminants escaping from the hood. Additionally, high face velocities can be annoying to the operator and can damage fragile apparatus in the hood. As the hazard level of the materials being handled and the resulting minimum face velocity increases, maintaining a safe face velocity becomes more difficult.
Another important consideration in the design of a fume hood system is the cost of running the system. There are three major areas of cost: the capital expenditure of installing the hood, the cost of power to operate the hood exhaust blower, and the cost of heating, cooling and delivering the "make-up air" which replaces the air exhausted from a room by the fume hood. For a hood operating continuously with an opening of 10 square feet and a face velocity of 100 fpm, the cost of heating and cooling the make-up air, could, for example, run as high as two thousand dollars per year in the northeastern United States. Where chemical work is done, large numbers of fume hoods may be required, resulting in the make-up air costs being a significant portion of the HVAC cost for the facility. For example, the Massachusetts Institute of Technology has approximately 650 fume hoods, most of which are in operation 24 hours a day.
Reliability is another important factor in the design of a fume hood system. It is important that the face velocity of a fume hood not be allowed to go below a certain level. The amount of air being exhausted from a hood may be decreased by many common occurrences: duct blockage, fan belt slippage or breakage, deterioration of the blower blades, especially where corrosive materials are being handled, motor overload, and other factors. A reduction in air flow reduces the face velocity, and it is important to take immediate steps when a low flow condition occurs to prevent escape of contaminants from the hood.
A conventional fume hood consists of an enclosure which forms five sides of the hood and a hood sash or sashes which slide horizontally and/or vertically to provide a variable-sized opening on the sixth side. In this type of hood, the amount of air exhausted by the hood blower is essentially fixed, and the face velocity increases as the area of the sash opening decreases. As a result, the sash must be left open an appreciable amount even when the hood is not being used by an operator to allow air to enter the hood opening at a reasonable velocity. However, as is discussed in U.S. Pat. Nos. 4,528,898 and 4,706,553, the amount of energy required to deliver "make-up air" may be reduced by monitoring the sash position, and thus the opening in the fume hood, and by adjusting the blower and thus the exhaust volume of the hood linearly in proportion to the change in opening size in order to achieve a substantially constant face velocity. In these patents, the fume hood opening was covered by a single sash which opened in the vertical direction.
However, there are at least two other styles of fume hood which have advantages for various applications. In one such style, two or more sashes are mounted to slide horizontally on at least two tracks, the tracks being located at the top and bottom of the sash opening. This design is advantageous for energy conservation purposes since the maximum hood opening required to gain access to a particular area in the hood is reduced, reducing the exhaust volume of the hood. With, for example, a two-track, two-sash design, the maximum opening would be fifty percent of the total opening area, thus reducing the maximum exhaust volume of the hood by fifty percent. Another advantage of the horizontal sash design is that a sash can serve as a safety shield for the operator to work behind.
Maximum flexibility in minimizing the open area of a fume hood while providing full access to the hood is achieved with the third type of design wherein sashes mounted on tracks for horizontal movement are in turn mounted on a sash frame which may be moved vertically.
However, to achieve a constant face velocity with a fume hood design which utilizes horizontal moving sashes, a new method is required to measure the open sash area. This is because the absolute position of the sashes is not sufficient information by itself to indicate the open sash area of the hood. Instead, it is the relative position of the two or more sashes of the hood which determine the total open sash area. Measuring the absolute position of each sash and using this information to generate the amount of overlap of the sashes, and hence the open area, is achievable but is awkward and complex. This is particularly true where many sashes are involved, such as fume hood utilizing four sashes on two tracks (this being a very common configuration). The complexity of measuring sash openings in this way is even greater for fume hoods which utilize sashes movable both horizontally and vertically.
A simpler approach for sash area measurement is one which involves direct measurement of hood opening, something which is not possible with the prior art configurations.
Further, the sensing devices in the prior systems have involved sensors having elements on both sides of the sashes. This results in one of the elements being inside the fume hood where it is subjected to contaminants which may frequently be corrosive and thus may adversely affect the life and reliability of the sensor. Further, since horizontal sashes involve two tracks, rather than a single track as in the vertical sash devices, greater separation may exist between the sensor elements. Where one element of the sensor is a source of electromagnetic radiation such as a magnet and the other element of the sensor is a detector of the electromagnetic radiation, such as a magnetic flux detector, the separation between these two elements required with a two-track configuration may be greater than the distance which the flux from the magnet can effectively operate the detector. A need therefore exists for improved techniques for sensing sash position in a fume hood and in particular for both improved sensor elements and improved techniques for utilizing such elements for use with sashes mounted for horizontal movement.